Spiders

Spiders are arachnids rather than insects. Their close relatives include ticks, mites and scorpions. Both spiders and insects belong to the arthropods, the largest group of animals on Earth. A typical spider has eight legs, a body divided into two parts, fangs, and silk-spinning organs.

Spiders play a valuable role in nature by feeding on insects and helping to keep pest numbers down. Although there are thousands of spider species, the vast majority pose little danger to people. Only a few can deliver a medically significant bite, and serious harm is uncommon, especially where antivenom is available.

How spider venom works#

Venom helps a spider subdue its prey, usually flies and other small insects. A spider may bite a person if it feels cornered or threatened, but most spiders cannot pierce human skin, and when they do, the venom often causes little reaction. The discomfort of a typical bite can usually be eased with a cold pack.

A small group of larger or more venomous spiders can cause more significant bites, and antivenom exists for some of them.

People most at risk#

A severe reaction to venom is more likely in babies, young children, older adults, and people with an existing health condition. Some people are also allergic to venom and may develop a severe allergic reaction (anaphylaxis), which is a medical emergency.

Recognising the symptoms#

Most bites produce only minor effects such as redness, itching, mild swelling or local pain. A more serious bite may cause increasing or spreading pain, nausea, vomiting, sweating, dilated pupils, or muscle spasms. In rare cases of severe envenomation, a person may become disoriented or confused, or lose consciousness.

If you are not sure how serious a bite is, see your doctor, go to your nearest emergency department, or dial your local emergency number (for example 911 in the US and Canada, 112 across the EU and many countries, 999 in the UK, or 000 in Australia) to call an ambulance.

First aid#

What to do depends on the kind of spider involved:

• For most spider bites: wash the area well with soap and water, then place a cold pack on it for about 15 minutes, reapplying if pain continues.
• For bites from large or highly venomous spiders that can cause severe, whole-body symptoms: apply a pressure bandage, immobilise the limb, and get urgent medical help.

Whatever the spider:

• Do not cut the bite or apply a tourniquet.
• Do not give the person food or drink.
• Keep them calm and as still as possible, and seek medical advice if in doubt.

Pressure bandaging and immobilisation#

This technique slows venom moving through the body. Firm bandaging compresses the lymph vessels near the bite, helping to keep venom at the puncture site, while keeping the limb still slows the spread further, because the lymphatic system depends on muscle movement to push fluid along.

For a bite on a limb:

• Place an elasticised roller bandage (roughly 10–15 cm wide) over the bite.
• Add a second bandage starting just above the fingers or toes and wrapping upwards as far as you can reach.
• Wrap it firmly, as you would for a sprain — a finger should not slide easily between bandage and skin.
• Splint the limb to hold it steady.
• Keep the person lying down and completely still.

If no bandages are available, improvise with torn towels or clothing. Note the time of the bite and when you applied the bandage. If you can, mark the bite site on the skin or bandage, or photograph it, and do not wash venom from the skin or clothes, as it may help identification. Stay with the person until help arrives.

Identifying the spider#

Knowing which spider was responsible can help medical staff choose the right treatment. A clear photograph, or a description of its size, colour, bulk and where it was when it bit, can all be helpful.

If you want to capture it safely:

• Use an empty container with a tight-fitting lid, such as a jar.
• Lower the container over the spider.
• Slide stiff cardboard underneath to seal it.
• Hold the cardboard in place, turn the container upright, and let the spider drop to the bottom.
• Remove the cardboard and secure the lid.

Never handle a spider you cannot identify.

Necrotising skin reactions#

Necrotising arachnidism describes skin inflammation and ulceration that has sometimes been blamed on spider bites. In rare cases the damage is extensive enough to need skin grafts. However, the connection between specific spiders and these wounds is uncertain, and research continues.

It is not understood why most bitten people recover with only minor symptoms while a very small number develop ulceration. Skin breakdown and tissue death have many other possible causes, including burns, poor circulation, unmanaged diabetes, and certain fungal or bacterial infections. Treatment may involve antibiotics, corticosteroids, removal of dead tissue, skin grafts, or other care directed by a doctor.

Preventing bites#

To lower your risk:

• Wear gloves, long trousers and shoes when gardening.
• Keep shoes on when walking around the garden.
• Probe any suspected web or hiding spot with a stick, never your hands.
• Shake out shoes and clothing before wearing them, and avoid leaving clothes on the floor.
• Tell children not to touch spiders.
• Never assume a spider in water is dead, as some can survive on an air bubble for many hours.

Keeping spiders out of the home#

Removing spiders from the garden may be impossible, but you can discourage them from coming indoors:

• Clear trees, shrubs and bushes away from doors and windows.
• Fit draught-strips to doors so spiders cannot crawl underneath.
• Put flyscreens on windows and vents.
• Avoid leaving shoes, clothing or equipment outdoors, and shake them out before bringing them in.
• Keep parked car windows closed so a spider does not surprise you while driving.

For persistent problems, ask a licensed pest control operator for advice.

Key points#

• Spiders are an important part of the ecosystem, preying on insects and keeping their numbers under control.
• Most spider bites are minor; only a small number of spiders can cause potentially harmful bites.
• Antivenom is available for some venomous spider bites.
• Immediate medical treatment is recommended if a bite causes severe or whole-body symptoms.
• Symptoms of a venomous spider bite depend on the species and can range from local pain to, rarely, loss of consciousness.

Where to get help#